1. Technical Field
The present invention relates to a CR oscillation circuit including a CR time constant circuit having a capacitor and a resistance element as a charge and discharge path of the capacitor.
2. Related Art
CR oscillation circuits each including a switching gate such as an inverter forming a loop for circulation of a signal and a CR time constant circuit inserted in the middle of the loop for delaying the signal are known. The oscillation frequency of the CR oscillation circuit depends on the time constant of the CR time constant circuit. Therefore, in order to keep the oscillation frequency of the CR oscillation circuit at a target frequency, it is necessary to minimize variations in time constant of the CR time constant circuit. However, the time constant of the CR time constant circuit may vary due to variations in manufacturing of the resistance element of the CR time constant circuit and the oscillation frequency of the CR oscillation circuit may shift from the target frequency. Accordingly, Patent Document 1 (JP-A-2002-33644) proposes a CR oscillation circuit in which, of a plurality of resistance elements, a resistance element serving as a charge and discharge path of a capacitor of the CR oscillation circuit may be selected by a transmission gate. According to the CR oscillation circuit, the resistance element as the charge and discharge path of the capacitor may be appropriately selected by the transmission gate, and thereby, even when there are variations in manufacturing in resistance values of the respective resistance elements, the resistance value of the charge and discharge path of the capacitor may be made as close to a target value as possible and the oscillation frequency of the CR oscillation circuit may be made closer to the target frequency.
In the above described CR oscillation circuit of Patent Document 1, an ON-resistance of the transmission gate fluctuates depending on a power supply voltage. Accordingly, under the technology of related art, a constant power supply voltage is supplied to the CR oscillation circuit by a stabilized power supply. However, in the CR oscillation circuit of Patent Document 1, a threshold voltage of the field-effect transistor forming the transmission gate changes due to changes in ambient temperature. Therefore, the CR oscillation circuit of Patent Document 1 has a problem that the ON-resistance of the transmission gate changes depending on the ambient temperature, the time constant of the CR time constant circuit changes, and the oscillation frequency of the CR oscillation circuit changes.